Index: src/applications/diffusion/engine/DiffusionCommitHookEngine.php
===================================================================
--- src/applications/diffusion/engine/DiffusionCommitHookEngine.php
+++ src/applications/diffusion/engine/DiffusionCommitHookEngine.php
@@ -1004,6 +1004,9 @@
   }
 
   public function loadChangesetsForCommit($identifier) {
+    $byte_limit = HeraldCommitAdapter::getEnormousByteLimit();
+    $time_limit = HeraldCommitAdapter::getEnormousTimeLimit();
+
     $vcs = $this->getRepository()->getVersionControlSystem();
     switch ($vcs) {
       case PhabricatorRepositoryType::REPOSITORY_TYPE_GIT:
@@ -1015,8 +1018,10 @@
             'user' => $this->getViewer(),
             'commit' => $identifier,
           ));
+
         $raw_diff = DiffusionRawDiffQuery::newFromDiffusionRequest($drequest)
-          ->setTimeout(5 * 60)
+          ->setTimeout($time_limit)
+          ->setByteLimit($byte_limit)
           ->setLinesOfContext(0)
           ->loadRawDiff();
         break;
@@ -1027,15 +1032,29 @@
         // the "--diff-cmd" flag.
 
         // For subversion, we need to use `svnlook`.
-        list($raw_diff) = execx(
+        $future = new ExecFuture(
           'svnlook diff -t %s %s',
           $this->subversionTransaction,
           $this->subversionRepository);
+
+        $future->setTimeout($time_limit);
+        $future->setStdoutSizeLimit($byte_limit);
+        $future->setStderrSizeLimit($byte_limit);
+
+        list($raw_diff) = $future->resolvex();
         break;
       default:
         throw new Exception(pht("Unknown VCS '%s!'", $vcs));
     }
 
+    if (strlen($raw_diff) >= $byte_limit) {
+      throw new Exception(
+        pht(
+          'The raw text of this change is enormous (larger than %d '.
+          'bytes). Herald can not process it.',
+          $byte_limit));
+    }
+
     $parser = new ArcanistDiffParser();
     $changes = $parser->parseDiff($raw_diff);
     $diff = DifferentialDiff::newFromRawChanges($changes);
Index: src/applications/diffusion/herald/HeraldPreCommitContentAdapter.php
===================================================================
--- src/applications/diffusion/herald/HeraldPreCommitContentAdapter.php
+++ src/applications/diffusion/herald/HeraldPreCommitContentAdapter.php
@@ -34,6 +34,7 @@
         self::FIELD_DIFF_CONTENT,
         self::FIELD_DIFF_ADDED_CONTENT,
         self::FIELD_DIFF_REMOVED_CONTENT,
+        self::FIELD_DIFF_ENORMOUS,
         self::FIELD_REPOSITORY,
         self::FIELD_REPOSITORY_PROJECTS,
         self::FIELD_PUSHER,
@@ -75,6 +76,9 @@
         return $this->getDiffContent('+');
       case self::FIELD_DIFF_REMOVED_CONTENT:
         return $this->getDiffContent('-');
+      case self::FIELD_DIFF_ENORMOUS:
+        $this->getDiffContent('*');
+        return ($this->changesets instanceof Exception);
       case self::FIELD_REPOSITORY:
         return $this->getHookEngine()->getRepository()->getPHID();
       case self::FIELD_REPOSITORY_PROJECTS:
Index: src/applications/herald/adapter/HeraldCommitAdapter.php
===================================================================
--- src/applications/herald/adapter/HeraldCommitAdapter.php
+++ src/applications/herald/adapter/HeraldCommitAdapter.php
@@ -270,6 +270,14 @@
     return $this->affectedRevision;
   }
 
+  public static function getEnormousByteLimit() {
+    return 1024 * 1024 * 1024; // 1GB
+  }
+
+  public static function getEnormousTimeLimit() {
+    return 60 * 15; // 15 Minutes
+  }
+
   private function loadCommitDiff() {
     $drequest = DiffusionRequest::newFromDictionary(
       array(
@@ -278,7 +286,7 @@
         'commit' => $this->commit->getCommitIdentifier(),
       ));
 
-    $byte_limit = (1024 * 1024 * 1024); // 1GB
+    $byte_limit = self::getEnormousByteLimit();
 
     $raw = DiffusionQuery::callConduitWithDiffusionRequest(
       PhabricatorUser::getOmnipotentUser(),
@@ -286,7 +294,7 @@
       'diffusion.rawdiffquery',
       array(
         'commit' => $this->commit->getCommitIdentifier(),
-        'timeout' => (60 * 15), // 15 minutes
+        'timeout' => self::getEnormousTimeLimit(),
         'byteLimit' => $byte_limit,
         'linesOfContext' => 0,
       ));
Index: src/applications/herald/storage/HeraldRule.php
===================================================================
--- src/applications/herald/storage/HeraldRule.php
+++ src/applications/herald/storage/HeraldRule.php
@@ -17,7 +17,7 @@
   protected $isDisabled = 0;
   protected $triggerObjectPHID;
 
-  protected $configVersion = 25;
+  protected $configVersion = 26;
 
   // phids for which this rule has been applied
   private $ruleApplied = self::ATTACHABLE;